Friday, November 28, 2014

Messy Week Ahead

Here's what we know right now: The Sierra and Tahoe will have an active period of weather through midweek. The first snow begins this evening and continues overnight into tomorrow morning. Snow levels will drop behind the cold front that is trailing an upper level trough in the Pacific Northwest. The Sierra will see at least 6" of snow from this first storm above 7000' (Figure 1).

The second storm arrives early Sunday morning and persists into Sunday night. The 2nd storm won't be as strong or wet as the 1st storm, which means less snow. We are expecting at least 3" of snow above 7000' from the 2nd storm.

The final storm will arrive early next week, most likely Tuesday and continue into Wednesday. This storm appears to be wetter, but also initially warmer than the previous 2 storms. We will have a more in-depth analysis of this storm later this weekend.

A cold front approaches and moves through the Northern and Central Sierra tonight and tomorrow morning. Dewpoints are low, which will help cool the temperatures quicker and turn the rain to snow faster via the wet bulb effect (evaporative cooling). Warmer and drier air does lead to more evaporation, therefore, less snow reaches the ground. However, the air mass modification with the cold front and the associated lifting and cooling with the front will help mitigate this problem.

The snow levels will drop from ~9000' to ~7000' with the cold front and radiative cooling overnight (Figure 3). Snow levels will remain around 6000-7000' Saturday and into Sunday when the next shortwave moves through the Sierra.

With the cold front away from the main trough, it will weaken and relinquish to southerly flow on Sunday. This southerly flow will strengthen Sunday night and through Monday ahead of the larger upper level trough moving onshore from the eastern Pacific.

The models have been extremely inconsistent over the past week handling the change in the pattern. That is one of the reasons we waited to post until this evening. The precipitation totals that were predicted earlier this week were quite high, but have slowly come down throughout the week. The main trough was also much slower to move onshore, which caused some of the inconsistencies in the models and lowered the confidence of the forecast.